The invention is system, apparatus, and method (collectively the “system”) for displaying an image. More specifically, the system can effectively display an image with a wider field of view (“FOV”) to a user.
Image display technologies allow a human being to perceive an artificially created image, such as the image on a smart phone, tablet computer, television, arcade game screen, computer monitor, movie screen, or other similar devices. Such technologies can involve active screens, such as a visual display on a smart phone, or passive displays, such as a screen at a movie theater onto which images are projected onto. Some displays are large and far away like a scoreboard as a sports stadium while other displays may be small and close by such as viewing an image through an eye-piece or other form or near-eye display. Some displays can be positioned very close to the eyes of the viewer such as virtual reality goggles or virtual retinal displays worn on the head of the viewer which project an image directly onto the retina of the viewer.
Regardless of the specific process for creating and directing an image to a human viewer, the process for artificially creating an image that is visually perceived by a viewer is different in many respects from the process by which a human being visually perceives the real world. The real world is by default a 360° world. In the real world, light forming different portions of the viewer's field of view (“FOV”) can originate from different places, travelling substantially different paths, and even benefiting from different light sources. In an image display device, light is modulated to give the appearance of a field of view. The wider the FOV, the more challenging it can be to provide such an image in a realistic and effective manner. The challenges can be even more pronounced in the context of near-eye displays such as displays involving eye pieces or head-mounted image display devices.
I. The “Exit Pupil” of an Optical System such as an Image Display Device
The “exit pupil” is a term of art in the field of optics. It does not pertain to human eye. The exit pupil is sometimes referred to as the “eyepoint” or the “ramsden circle”. The exit pupil is the portion of a beam of light where the cross-section is the smallest. The exit pupil is a “virtual aperture” in an optical system. Only rays of light within the exit pupil can be perceived by the human viewer, and such perception is dependent on a sufficient alignment between the exit pupil of the device and the entrance pupil of the human eye.
II. Drift or Displacement in the Exit Pupil
As illustrated in FIG. 1b, in an idealized description of optics, the exit pupil is located a single point. In practical applications. In the real world devices there is some measure of displacement or drift. That displacement is often referred to as “exit pupil drift”, “optic drift” or simply “displacement” or “drift”. The exit pupil drift of tends to increase as the FOV increases, and as the device itself is reduced in size. Thus, exit pupil drift is particularly problem in a near-eye display such as an eye-piece based device or a head-mounted display such as virtual reality goggles. FIGS. 1c-1e illustrate examples of exit pupil drift that would result loss of color and intensity within different areas of the FOV. In contrast, FIGS. 1f-1h illustrate examples of exit pupil drift that will have minimal if any impact on the user experience with the image display device.
Different image display technologies can make it relatively easier or harder to provide an image with a relatively more expanded field of view. For example, it is well known that plasma TVs have a substantially wider viewing angle than LCD TVs. Although manufacturers of such TVs may assert a viewing angle of 178°, at least one independent reviewer of LED TVs in 2014 found effective viewing angles of between 36° and 74°. Unlike light in the real world which originates from a variety of sources, travels a variety of paths, and approaches the eye from a variety of angles, the light from a television set originates from substantially the same place, travels substantially the same path, and approaches the eye from substantially the same angle.
If the exit pupil drift in a displayed image is too substantial, it becomes impossible to properly align the exit pupil with the entrance pupil, i.e. the pupil in the eye of the viewer.
III. Alignment Between the Exit Pupil and the Entrance Pupil
Anyone who has struggled to view an image through an eyepiece can understand the importance of an exit pupil that is properly aligned with the entrance pupil, i.e. the pupil of the human eye viewing the displayed image. This properly couples the display device to the eye and avoids “vignetting” which is a reduction in the brightness or color of an image on the periphery when compared to the image center.
Maintaining a proper alignment between the exit pupil of the device and the entrance pupil of the eye can be particularly challenging in the context of head-mounted display device because of the movement of the eye of the viewer. Substantial exit pupil drift makes it that much harder for the viewer of the image to sustain that alignment. Head-mounted display devices such as a VRD visor apparatus are intended to allow the eye to move and focus on different areas within the displayed image.
IV. Image Display Devices are Far More Limiting than the Human Eye
In terms of field of view, many image display devices are far more limiting than the inherent capabilities of the human eye. The field of view of a typical human eye is approximately 95° outwards from the nose, 75° in a downwards direction, 60° inwards towards the nose, and 60° upwards. Human beings have an almost 180° forward-facing horizontal field of view. Eyeball rotation can expand the horizontal field of view to almost as high as 270°.
All of the numbers referenced above represent the maximum capabilities of a typical human eye viewing images in the real world.
The world of image display devices is far more limiting. Some television sets provide an effective horizontal FOV of less than 45°. Near eye displays such as eye-piece based devices and head-mounted displays involve substantial exit pupil drift at about 40° or greater.
V. How the Eye Works
The outer surface of a human eye is the cornea. Light enters the eye through the cornea, which protects the interior portions of the eye while allowing light to enter the eye so that it can be seen. The cornea provides most of the focusing power of the eye, but that focus is fixed and cannot be adjusted by the cornea. The cornea possesses a curved and transparent surface. The cornea is typically about 8 mm in radius.
Light then passes from the cornea through the pupil, an adjustable opening in the center of the iris, which is the colored portion of the eye. Movement of the pupil (and iris) allows a human being to focus attention at different areas within the field of view. The pupil can also expand or contract to control the amount of light that passes through the pupil. Normal pupil size is typically between 2.0 mm and 5.0 mm.
The pupil of a human eye is small for the purposes of alignment with the exit pupil of an optical system. If properly aligned with the human eye, an image display device could create a more expansive FOV that could be effectively experienced by the viewer. However, the entrance pupil of the human eye is small, and a material magnitude of exit pupil drift renders prior art attempts at an expanded FOV undesirable for most viewers.